U.S. patent application number 11/142112 was filed with the patent office on 2006-03-16 for linear actuator.
Invention is credited to Stephen Harlow Davies, Mark Anthony Guy, Michael Paul Somerfield, David Roy Tucker.
Application Number | 20060054016 11/142112 |
Document ID | / |
Family ID | 34941527 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060054016 |
Kind Code |
A1 |
Davies; Stephen Harlow ; et
al. |
March 16, 2006 |
Linear actuator
Abstract
A linear hydraulic actuator comprises a hollow main piston
axially moveable in an elongate cylinder under the influence of
hydraulic pressure, the main piston defining a bore within which a
seal member is provided, the seal member forming a sliding seal
with the bore of the main piston.
Inventors: |
Davies; Stephen Harlow;
(Shifnal, GB) ; Guy; Mark Anthony; (Codsall,
GB) ; Somerfield; Michael Paul; (Stoke-on-Trent,
GB) ; Tucker; David Roy; (Craven Arms, GB) |
Correspondence
Address: |
ANDRUS, SCEALES, STARKE & SAWALL, LLP
100 EAST WISCONSIN AVENUE, SUITE 1100
MILWAUKEE
WI
53202
US
|
Family ID: |
34941527 |
Appl. No.: |
11/142112 |
Filed: |
June 1, 2005 |
Current U.S.
Class: |
92/109 |
Current CPC
Class: |
F15B 15/264 20130101;
F15B 15/1409 20130101; F15B 15/1466 20130101 |
Class at
Publication: |
092/109 |
International
Class: |
F01B 31/00 20060101
F01B031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 2, 2004 |
GB |
GB 0412255.2 |
Jun 29, 2004 |
GB |
GB 0414458.0 |
Dec 17, 2004 |
GB |
GB 0427687.9 |
Claims
1. A linear hydraulic actuator comprising a hollow main piston
axially moveable, through a range of movement, in an elongate
cylinder under the influence of hydraulic pressure, the main piston
defining a bore within which a seal member is provided, the seal
member forming a sliding seal with the bore of the main piston.
2. An actuator according to claim 1, wherein the seal member is
axially fixed relative to the cylinder.
3. An actuator according to claim 2, wherein the seal member is
attached to an end of the cylinder by an axially extending
stem.
4. An actuator according to claim 1, wherein the seal member
comprises a supplementary piston slidably received within said main
piston and movable longitudinally relative to said cylinder between
a rest position and a limit position, said supplementary piston and
said main piston including respective abutment surfaces which
co-act when both the main piston and the supplementary piston are
in their respective rest positions and hydraulic pressure is
applied to said supplementary piston to move it from its rest
position towards its limit position, said abutment of said surfaces
of said supplementary piston and said main piston being such that
during movement of the supplementary piston from its rest position
towards its limit position said supplementary piston pushes the
main piston in its movement from its rest position towards its
actuated position, whereafter, when movement of said supplementary
piston is arrested at its limit position said main piston can
continue to move relative to said cylinder towards its actuated
position sliding relative to said supplementary piston.
5. An actuator according to claim 4, wherein said supplementary
piston is slidably supported on an elongate element axially fixed
with respect to said cylinder, and said element and said
supplementary piston include an abutment surface defining said
limit position of said supplementary piston.
6. An actuator according to claim 5, wherein said elongate element
is hollow, and provides a communication path whereby hydraulic
fluid entering the elongate element in use can return to a low
pressure side of an associated hydraulic system.
7. An actuator according to claim 1, wherein the co-operation of
said main piston with said cylinder defines, within said cylinder,
first and second chambers on opposite sides respectively of said
main piston, said main piston exposing a larger surface area to
said first chamber than to said second chamber so that application
of the same hydraulic pressure to both chambers urges the main
piston to move in a direction relative to said cylinder to reduce
the volume of said second chamber.
8. An actuator according to claim 1, wherein the seal member and
bore of the main piston together define a closed chamber, and a
fluid flow path is provided between said closed chamber of said
main piston and a hydraulic fluid supply line, said fluid flow path
including a flow restrictor whereby the rate at which hydraulic
fluid can enter said closed chamber of said main piston from said
supply line is less than the rate at which the volume of said
closed chamber increases as said piston is moved over at least part
of the range of movement of the main piston.
9. An actuator according to claim 8, wherein a non-return valve is
associated with said fluid flow path so that during movement of
said piston relative to said cylinder to discharge hydraulic fluid
from the closed chamber through said fluid flow path, said valve
opens so that fluid discharged from the closed chamber is not
required to flow through said restrictor.
10. An actuator according to claim 9, wherein said seal member is
carried by a rod disposed coaxially within said hollow piston and
secured to the cylinder.
11. An actuator according to claim 10, wherein said fluid flow path
includes a passage extending through said rod.
12. An actuator according to claim 10, wherein said non-return
valve is carried by said rod.
13. An actuator according to claim 8, further comprising a
change-over valve operable in a first position to connect the
supply line to a supply of hydraulic fluid under pressure, and
operable in a second position to connect said supply line to low
pressure to permit discharge of hydraulic fluid from the
actuator.
14. An actuator according to claim 9, wherein said restrictor and
said non-return valve are connected hydraulically in parallel with
one another.
15. An actuator according to claim 9, wherein said restrictor and
said non-return valve are defined by a common component in the form
of a non-return valve which leaks in its closed position to permit
a restricted flow of hydraulic fluid from said supply line to said
closed chamber.
16. An actuator according to claim 1, further comprising a
synchronisation mechanism for synchronising movement of the main
piston with movement of one or more pistons of one or more other
actuators coupled to the synchronisation mechanism.
17. An actuator according to claim 16, wherein the synchronisation
mechanism comprises a rotatable tube mounted axially within the
cylinder and extending into the bore of the hollow main piston,
wherein the rotatable member has a thread that engages a
corresponding thread on the bore of the main piston such that
movement of the main piston causes rotation of the tube.
Description
[0001] The present invention relates to a linear hydraulic
actuator.
[0002] Linear hydraulic actuators are used in a wide range of
mechanical applications. They generally comprise a piston that is
axially moveable under hydraulic pressure within a cylinder. In
many applications the piston is connected to a shaft which extends
axially through a sealed end wall of the cylinder.
[0003] One example of an application of a linear hydraulic actuator
is on aircraft for actuation of thrust reversers. The exact nature
of the engine thrust reverser system is not of importance to an
understanding of the present invention. It will be recognised that
gas turbine engine thrust reverser systems include movable
elements, usually in the form of cowls or doors, which when
deployed reverse the engine thrust to assist in deceleration of the
aircraft. For convenience, throughout the remainder of this
specification, the movable elements will be referred to as thrust
reverser cowls. During normal operation a thrust reverser system
will only be actuated when the aircraft is decelerating on the
runway, and the gas turbine engine is operative. It is known that
in thrust reverser systems, the initial movement of the thrust
reverser cowls from their rest position, towards their deployed
position is resisted by the air/gas flow around/through the engine,
while at a later stage in the deployment the air/gas flow may
actually assist the deployment movement of the cowls.
[0004] Linear hydraulic actuators for use in aircraft engine thrust
reversers are designed to be of high strength and, as far as
possible, of light weight. It is known to provide the actuator with
a hollow piston assembly comprising a piston head and hollow piston
rod and to accommodate components of an actuator synchronisation
mechanism within the hollow bore of the piston assembly.
[0005] A hollow piston assembly of the above type is typically
located within an hydraulic cylinder such that separate hydraulic
chambers are formed on either side of the piston head. The piston
rod transmits movement of the piston assembly to an external
component to be moved by the actuator. As the piston rod extends
from one face of the piston head the piston head presents different
cross-sectional areas in the respective chambers to either side of
the piston head such that when, in use, a high pressure fluid
supply is connected to both hydraulic chambers a net force
determined by the difference in piston areas exposed to said high
pressure fluid, will be developed to displace the piston assembly
within the cylinder.
[0006] The volume of pressurised fluid required to extend an
hydraulic actuator of the above hollow piston assembly design may
be reduced by connecting the two hydraulic chambers together, such
that fluid displaced from one chamber can flow into the other
chamber as the piston moves due to the force developed as a result
of the different cross-sectional areas of the piston exposed to the
pressurised fluid in the respective chambers. The volume of
pressurised fluid required to be supplied from a high pressure
fluid pump is thus equal to the difference in cross-sectional areas
of the piston multiplied by the linear distance moved by the piston
within the hydraulic cylinder. The rate of flow of fluid, which
determines the size of the associated high pressure fluid pump, is
dependent on the speed at which the piston is displaced.
[0007] According to the present invention there is provided a
linear hydraulic actuator comprising a hollow piston axially
moveable in an elongate cylinder under the influence of hydraulic
pressure, the piston defining a bore within which a seal member is
provided, the seal member forming a sliding seal between the
cylinder and the bore of the piston.
[0008] The seal member may be attached to an opposing end of the
cylinder by an axially extending stem.
[0009] The advantages of the actuator of the present invention are
apparent from a consideration of its operation. With the piston in
a retracted position at one end of its stroke, when pressurised
hydraulic fluid is introduced to the cylinder, the piston moves so
that the bore slides over the seal member. This means that as the
tubular piston moves, the space inside the bore beyond the seal
member is no longer filled with pressurised hydraulic fluid.
Consequently, the volume of pressurised hydraulic fluid required
for actuation is substantially reduced when compared with a known
tubular piston design, and, for a given hydraulic system, the time
required for actuation is reduced. As a further consequence, the
diameter of the piston can be increased to meet the demands of
piston strength without any significant increase in the volume of
hydraulic fluid required to actuate the piston.
[0010] The cylinder may be further provided with a synchronisation
mechanism for synchronising movement of the piston with movement of
one or more pistons of one or more actuators coupled to the
synchronisation mechanism.
[0011] The synchronisation mechanism may comprise a rotatable tube
mounted axially within the cylinder and extending into the bore of
the tubular piston, wherein the rotatable member has a thread that
engages a corresponding thread on the bore of the tubular piston
such that movement of the piston causes rotation of the tube.
[0012] It is an advantage that the use of a tubular piston together
with a rotatable tube synchronisation mechanism can be deployed
without any detrimental increase in the volume of hydraulic fluid
required to actuate the piston.
[0013] The difference in piston cross-sectional areas provides a
predetermined driving force to the piston at the operating pressure
of the hydraulic system and is sized to provide a force in excess
of that anticipated to be required in normal operation of the
component to which the linear hydraulic actuator is attached in
use. As, in most circumstances and over a significant length of the
actuator stroke, a far lower force is required than is available
the actuator consumes a greater volume of pressurised hydraulic
fluid than necessary, resulting in a requirement for a larger
capacity pump than would otherwise be required.
[0014] As the actuator is operated by the supply of pressurised
hydraulic fluid only to an annular area of the piston assembly, the
volume and flow rate of high pressure hydraulic fluid which is
needed to operate the actuator is reduced, and a pump of
correspondingly reduced capacity can be utilised.
[0015] Although ideal for many applications, such an arrangement
suffers the disadvantage, particularly notable in an aircraft
engine thrust reverser environment, that for a constant operating
pressure, it generates the same output force throughout the whole
of its operating stroke. In many aircraft thrust reverser
environments the initial load which the actuator must overcome to
deploy the thrust reverser is high, but after deployment commences
the load which the actuator must overcome is reduced dramatically
as air flow over the thrust reverser assembly aids deployment of
the thrust reverser. It follows therefore that the actuator will be
designed to produce sufficient force to overcome the initial
deployment loads, and thus may be oversized for the remainder of
the operating stroke of the actuator.
[0016] Rather than being connected to the opposing end of the
cylinder by a stem, the seal member may comprise a supplementary
piston slidably received within said main piston and movable
longitudinally relative to said cylinder between a rest position
and a limit position, said supplementary piston and said main
piston including respective abutment surfaces which co-act when
both the main piston and the supplementary piston are in their
respective rest positions and hydraulic pressure is applied to said
supplementary piston to move it from its rest position towards its
limit position, said abutment of said surfaces of said
supplementary piston and said main piston being such that during
movement of the supplementary piston from its rest position to its
limit position said supplementary piston assists the main piston in
its movement from its rest position towards its actuated position,
whereafter, when said supplementary piston is arrested at its limit
position said main piston can continue to move relative to said
cylinder towards its actuated position sliding relative to said
supplementary piston.
[0017] Such an arrangement is advantageous in that it permits the
generation of an increased actuation force during the initial
movement of the actuator from a rest position towards a deployed
position.
[0018] Preferably said supplementary piston is slidably supported
on an elongate element axially fixed with respect to said cylinder,
and said element and said supplementary piston including an
abutment surface defining said limit position of said supplementary
piston.
[0019] Conveniently said element and said supplementary piston
include an abutment surface defining said rest position of said
supplementary piston.
[0020] Desirably said elongate element is hollow, and provides a
drain path whereby hydraulic fluid entering the elongate element in
use can drain back to a low pressure side of an associated
hydraulic system.
[0021] Preferably the co-operation of said main piston with said
cylinder defines, within said cylinder, first and second chambers
on opposite sides respectively of said main piston, said main
piston exposing a larger surface area to said first chamber than to
said second chamber so that application of the same hydraulic
pressure to both chambers urges the main piston to move in a
direction relative to said cylinder to reduce the volume of said
second chamber.
[0022] It is recognised that use of such an additional piston in
the actuator cannot accommodate an increased load which the
actuator could unexpectedly be called upon to overcome at a point
in the operating stroke of the actuator beyond the stroke of the
additional piston, and it is another object of the present
invention to provide an actuator in which the volume of high
pressure hydraulic fluid needed to operate the actuator is
minimised while at the same time providing the facility to generate
higher output forces at any point in the stroke of the actuator
should the actuator be called upon to do so.
[0023] A known method of providing increased actuation force is to
increase the hydraulic pressure applied to the actuator. However,
where an increase in available hydraulic pressure is not possible
then it has been proposed to increase the piston diameter and thus
the effective piston area of the piston and cylinder arrangement of
the hydraulically operated linear actuator in order to provide the
necessary initial force. Such a solution of course produces an
actuator which has increased force throughout the whole of the
range of movement of the actuator. Moreover, increasing the piston
diameter involves an increase in the size, and thus the mass of the
actuator and consequent upon this there is needed an increase in
the volume and/or flow rate of hydraulic fluid which must be
supplied to the actuator to perform an actuation stroke within the
required operating time. Usually an increase in the volume of
hydraulic fluid required involves an increase in the mass of the
associated hydraulic fluid pump, and such mass increases are
generally not acceptable in aircraft systems.
[0024] U.S. Pat. No. 5,941,158 discloses an hydraulic linear
actuator in which an increase in the force generated during initial
actuation is achieved without the disadvantages mentioned above,
through the use of a supplementary piston which augments the force
provided by the main piston of the actuator during initial movement
of the actuator from a rest position towards a deployed position.
It can be seen that the actuator of U.S. Pat. No. 5,941,158
utilises a supplementary piston of annular form sliding in an end
closure member of the cylinder and the main piston of the actuator,
which carries most of the loads and vibration of operation of the
associated thrust reverser system in use, sliding in the
supplementary piston. The sliding interfaces of the main and
supplementary pistons and the end closure member are sealed by
concentric seals and it is found that it is difficult, given the
inherent compliance necessary in the seals, to maintain accurate
control over the tolerances in such an assembly with the attendant
risk of leakage of hydraulic fluid past the seals and loss of the
fluid externally of the actuator. It is a further object of the
present invention to provide an hydraulically operated linear
actuator in which an increased operating force is available during
initial deployment movement of the actuator, and wherein the
actuator construction is more suited to prolonged operation at high
operating pressures.
[0025] The seal member and bore of the main piston may together
define a closed chamber, and, a fluid flow path may be provided
between said inner, closed, end chamber of said hollow piston
assembly and a hydraulic fluid supply line, said fluid flow path
including a flow restrictor whereby the rate at which hydraulic
fluid can enter said inner, closed, end chamber of said hollow
piston assembly from said supply line is less than the rate at
which the volume of said inner, closed end chamber increases as
said piston assembly is moved during normal operation.
[0026] Preferably a non-return valve is associated with said fluid
flow path so that during movement of said piston assembly relative
to said cylinder to discharge hydraulic fluid from the interior of
the piston assembly through said path, said valve opens so that
fluid discharged from the interior of said hollow piston assembly
is not required to flow through said restrictor.
[0027] Desirably said seal member is carried by a rod disposed
coaxially within said hollow piston assembly and secured to the
cylinder.
[0028] Conveniently said fluid flow path includes a passage
extending through said rod.
[0029] Desirably said non-return valve is carried by said rod.
[0030] Conveniently said passage through said rod interconnects
said pressure chamber and said end chamber.
[0031] Preferably the linear hydraulic actuator includes a
change-over valve operable in a first position to connect said
supply line to a supply of hydraulic fluid under pressure, and
operable in a second position to connect said supply line to low
pressure to permit discharge of hydraulic fluid from the
actuator.
[0032] Conveniently said restrictor and said non-return valve are
connected hydraulically in parallel with one another.
[0033] Conveniently said restrictor and said non-return valve are
defined by a common component in the form of a non-return valve
which leaks in its closed position to permit a restricted flow of
hydraulic fluid from said supply line to said end chamber.
[0034] Desirably said hollow piston assembly includes a piston rod
protruding from one axial end of said cylinder for connection to a
component to be actuated by the actuator.
[0035] The invention will further be described, by way of example
only, with reference to the accompanying drawings, in which:
[0036] FIG. 1 shows a known linear hydraulic actuator that is used
in an aircraft thrust reverser actuation system;
[0037] FIG. 2 shows a linear hydraulic actuator in accordance with
an embodiment of the invention for use in a similar application to
the actuator of FIG. 1;
[0038] FIG. 3 is a diagrammatic cross-sectional representation of
an hydraulically operable linear actuator in accordance with
another example of the present invention;
[0039] FIG. 4 is a view similar to FIG. 3 of part of an
hydraulically operable linear actuator in accordance with another
example of the present invention;
[0040] FIG. 5 a diagrammatic representation of a linear hydraulic
actuator connected for operation in an extending mode;
[0041] FIG. 6 is a view similar to FIG. 5 but showing connections
to the actuator for operation in a retraction mode; and
[0042] FIG. 7 is a view similar to FIG. 5 of a modification.
[0043] Referring to FIG. 1, a linear hydraulic actuator has a body
10, which defines a cylinder 12 having an inner cylindrical surface
15. A piston 14 having a large diameter cylindrical section 16 fits
within the cylinder 12 and is separated from the inner cylindrical
surface 15 by a first slidable circumferential seal 26. The piston
14 divides the space inside the cylinder 12 into a first chamber 11
and a second chamber 13.
[0044] The piston 14 also has a tubular section 20 that extends
through an end wall 19 of the housing 10. A second slidable seal 28
is provided between the end wall and the tubular section 20. Thus,
the piston 14 has an inboard end having an annular surface 18, and
an outboard end 32 for coupling to a thrust reverser cowl. The
tubular section 20 of the piston 14 has an open inboard end 21
within the cylinder, a bore 22 and a closed outboard end 23. The
effective thrust surface of the piston exposed to the fluid
pressure within the first chamber 11 is the full cross-sectional
area of the cylinder.
[0045] A rotatable tube 34 is mounted in a pair of bearings 36, 38
and extends axially inside the cylinder 12 and into the open end 21
of the tubular section 20 of the piston 14. The rotatable tube 34
carries a coarse-pitch male thread 40, which engages a
corresponding female thread 42 on a section of the bore 22 of the
tubular section 20. A mechanical linkage 46 engages teeth 44 formed
on the rotatable tube 34 between the bearings 36, 38. The linkage
46 extends from the actuator 10 to engage a corresponding rotatable
tube in a second identical actuator.
[0046] In use, when hydraulic fluid under pressure is introduced
into the cylinder 12 in the first chamber 11, the fluid pressure
exerts a force against the piston 14 causing it to move (to the
right as shown in FIG. 1) for operation of the thrust reverser
member. As the piston 14 moves, the volume of the space to be
filled with hydraulic fluid expands. This space includes not only
the swept volume of the annular surface 18 in the region
surrounding the rotatable tube 34, but also includes the volume of
the space inside the bore 22 of the piston 14. A substantial
quantity of hydraulic fluid must thus be pumped under pressure into
the actuator 10 in order to cause operation thereof within the
required operating time.
[0047] Movement of the piston 14 causes rotation of the rotatable
tube 34 by way of the threaded engagement between the male thread
40 and the female thread 42. Rotation of the rotatable tube 34
activates movement of the linkage 46. The linkage 46 ensures that
movement of the piston of the second actuator is synchronised with
(i.e. identical to) the movement of the piston 16. This ensures
that the thrust reverser is actuated with an equal movement at two
(or more) points of actuation so as to prevent distortion of the
thrust reverser member.
[0048] The piston is retracted back into the cylinder by removing
the source of pressurised hydraulic fluid to the first chamber 11
and by introducing pressurised fluid only into the second chamber
13.
[0049] Referring to FIG. 2, equivalent features are provided with
equivalent reference numerals to those used for the actuator of
FIG. 1. An actuator 50 is provided with a seal member 52 disposed
internally of the tubular piston 14. The seal member 52 has a
circumferential seal 54, which forms a slidable seal against the
bore 22 of the piston 14. An axially extending stem 56 extends from
the seal member 52 to an opposing end 48 of the actuator 50. The
seal member 52 divides the space inside the bore 22 of the piston
14 so as to provide an outboard chamber 57. An opening 58 is
provided through the piston 14 so that the outboard chamber 57 is
open to the surrounding atmospheric pressure.
[0050] In use, as with the actuator of FIG. 1, when hydraulic fluid
under pressure is introduced into the cylinder 12 in the first
chamber 11, the fluid pressure exerts a force against the annular
surface 18 of the piston 14 to move it (to the right as shown in
FIG. 2) for operation of the thrust reverser cowl. The piston 14
slides past the second slidable seal 28 and the circumferential
seal 54 on the seal member 52. Unlike the actuator of FIG. 1, the
volume of the chamber 11 expands only by the swept volume of the
annular surface 18, on movement of the actuator. The volume of
pressurised hydraulic fluid within that part of the bore 22 of the
piston 14 to the left of the seal member 52 (in the orientation
illustrated) decreases as the piston extends to the right (as shown
in FIG. 2) because the seal member 52 remains fixed while the
piston moves past it. Thus a reduced quantity of hydraulic fluid is
required to operate the actuator 50, when compared with the
actuator 10 of FIG. 1. Consequently, for a given rate of high
pressure supply, the time required to operate the actuator is
reduced.
[0051] Retraction of the piston 14, and operation of the
synchronisation mechanism occurs in the same way as for the
actuator of FIG. 1. However, it should be borne in mind that the
principles of the present invention are not restricted to the
application of thrust reverser actuators, but apply equally to any
linear hydraulic actuator that employs a tubular piston
construction. It is also apparent that the actuator need not be
provided with a second hydraulic chamber 13 for retracting the
piston. Instead, retraction may be effected by removing the
pressure of hydraulic fluid from the first chamber 11 and allowing
external forces to push the piston 14 back into the cylinder
12.
[0052] Referring next to FIG. 3 there is illustrated a linear
actuator including an elongate hydraulic cylinder 111 of circular
cross-section, the cylinder 111 including opposite axial end
closure members 112, 113. Adjacent each closure member 112, 113 the
wall of the cylinder 111 includes a respective port 114, 115
through which hydraulic fluid can flow into and out of the
cylinder.
[0053] Slidably received within the cylinder 111 is an elongate
piston 116 including an annular piston head 117 and integral
therewith, or rigidly secured thereto, a hollow elongate piston rod
118. The external diameter of the piston head 117 and the internal
diameter of the cylinder 111 are such that the piston head 117 is a
close sliding fit within the cylinder, and the piston head 117
includes an annular external sealing ring 120, sealing the sliding
interface of the piston 116 and cylinder 111. The piston rod 118 is
of elongate rectilinear form and is disposed with its longitudinal
axis coincident with the longitudinal axis of the cylinder 111. The
piston rod 118 is smaller in diameter than the internal diameter of
the cylinder and so an annular clearance exists between the piston
rod 118 and the inner wall of the cylinder 111. The cylinder end
closure member 113 is annular, and the piston rod 118 extends
within the cylinder 111, and protrudes through the central aperture
of the closure member 113 for mechanical connection to the movable
cowl assembly of an associated gas turbine engine thrust reverser
system. Conveniently the free, outer end of the piston rod 118 has
a universal ball connection indicated diagrammatically at 119. The
surface of the central bore of the closure member 113 is formed
with a pair of parallel circumferential grooves receiving sealing
rings 121 which seal the sliding interface of the cylindrical outer
surface of the piston rod 118 and the closure member 113. It will
be appreciated that, as thus far described, the arrangement is very
similar to that of FIG. 2.
[0054] Anchored to the closure member 112 and extending coaxially
within the cylinder 111 and piston 116 is an elongate tubular
element 122 of circular cross-section. The element 122 extends
through the closure member 112 and so the central passage 123 of
the tubular element 122 is accessible at the exterior of the
closure member 112. The element 122 extends through the full axial
length of the cylinder 111 and terminates within the piston rod 118
adjacent the outer end of the closure member 113. The free end
region of the element 122 is formed with a neck region 124 of
reduced external diameter, the neck region 124 being defined
between opposed, spaced, radial shoulders 125, 126 of the element
122.
[0055] A seal member in the form of an annular supplementary piston
127 is slidably mounted on the neck region 124 of the element 122,
and can slide relative to the element 122 between a rest position
and a limit position defined respectively by the abutment of the
respective ends of the piston 127 with the shoulders 125 and 126.
An internal sealing ring 128 seals the sliding interface of the
piston 127 and the neck region 124 of the element 122, and
furthermore the piston 127 is in sliding engagement with the inner
cylindrical surface of the hollow piston rod 118, the sliding
interface of the supplementary piston 127 and the interior of the
piston rod 118 being sealed by annular sealing rings 129.
Internally, adjacent its outer end, the piston rod 118 is formed
with a radially inwardly directed shoulder 131 against which one
axial end of the supplementary piston 127 can abut in use.
[0056] The piston head 117 divides the interior of the cylinder 111
into first and second chambers 132, 133. The actuator has a rest
position in which the piston 116 is retracted within the cylinder
111. In order to extend the piston hydraulic fluid under pressure
is admitted to the cylinder 132 to displace the piston 116 to the
right of the position shown in FIG. 3 to increase the amount by
which the piston protrudes from the closure member 113. The
actuator has an actuated position (not shown) in which the piston
head 117 is close to, or actually abuts, the inner face of the
cylinder closure member 113.
[0057] It should be recognised that the relative positions of the
main piston 116 and supplementary piston 127, which are depicted in
FIG. 3, will not normally exist in practice, and with the main
piston 116 in its retracted, rest position the right-hand end of
the supplementary piston 127 would normally abut the shoulder 131
of the piston rod 118.
[0058] Assuming therefore that the supplementary piston 127 is
abutting the shoulder 131, then the application of hydraulic fluid
under pressure to the chamber 132 through the port 114 exposes the
left-hand end face of the piston head 117, the left-hand end face
of the piston rod 118, and the left-hand end face of the
supplementary piston 127 to hydraulic fluid under pressure, thus
driving the main piston 116 to the right, to extend the piston. The
force generated is of course a function of the hydraulic pressure
difference across the assembly of pistons, and the area of the
pistons exposed to the pressure.
[0059] The piston 116 and the piston 127 will continue to move to
the right as a unit until the supplementary piston 127 abuts the
shoulder 126 of the element 122, arresting the piston 127 against
further axial movement to the right relative to the element 122 and
the cylinder 111. Although hydraulic pressure still acts against
the left-hand face of the supplementary piston 127 this is of no
effect since the supplementary piston 127 cannot move any further
to the right. However the left-hand face of the piston head 117 and
piston rod 118 continue to be subjected to hydraulic pressure and
the piston 116 thus continues to move, sliding relative to the now
arrested supplementary piston 127. It will be recognised that
because the exposed surface area of the supplementary piston 127 is
no longer assisting the movement of the piston 116 then the force
generated by the actuator is reduced, without changing the pressure
of the hydraulic fluid, immediately the supplementary piston 127 is
arrested. After the supplementary piston 127 contacts the shoulder
126, further extension of the piston 116 will require a smaller
volume of hydraulic fluid to be supplied to the chamber 132 per
unit of extension than when the movement of the piston 116 was
assisted by the supplementary piston 127.
[0060] FIG. 3 shows that there is a relatively small distance
through which the piston 127 can move with the piston rod 118
before abutting the shoulder 126 of the element. 122. In practice
of course this distance will be set in accordance with the length
of the stroke of the piston 116 over which the assistance of the
supplementary piston 127 is required. This in turn will be
determined by the nature of the mechanism being actuated by the
actuator.
[0061] FIG. 3 shows a convenient hydraulic circuit for the
actuator. It can be seen that hydraulic fluid under pressure is
supplied from a high pressure source 134 through a changeover valve
135. The valve 135 also has a connection to low pressure 137 which
is also connected to the interior of the right hand end region of
the piston rod 118 through the passage 123 of the element 122. In a
first, actuation position of the valve 135 hydraulic fluid under
pressure is supplied simultaneously to both port 114 and port 115,
and so the same hydraulic pressure is applied to both opposite
exposed faces of the piston head 117. The effect of supplying the
same hydraulic fluid pressure to both chamber 132 and chamber 133
is to negate the effect on the piston 116 of the surface area of
the piston head 117. Nevertheless, the piston 116 will still be
moved to the right since the hydraulic pressure in the chamber 132
is applied also to the end face of the piston rod 118, and also,
during initial movement from the rest position, to the
supplementary piston 127.
[0062] Even when the supplementary piston 127 is arrested the
piston 116 is still biased to the right by the effect of the
hydraulic pressure on the end surface area of the piston rod 118.
It can be seen that the ports 114, 115 are actually interconnected
through the valve 135, and thus hydraulic fluid displaced from the
chamber 133 by movement of the piston 116 to the right flows to the
chamber 132 through the port 114 minimising the volume of hydraulic
fluid which needs to be supplied from the source 134 to operate the
actuator. Moreover as the main piston 116 moves relative to the
arrested supplementary piston 127 the increasing volume of the void
in the piston rod 118 to the right of the piston 127 is filled by
low pressure hydraulic fluid drawn into the void through the
passage 123.
[0063] When it is necessary to retract the piston 116 from its
actuated position back towards its rest position, the valve 135 is
moved to its second position in which hydraulic fluid under
pressure from the source 134 is applied only to the chamber 133
through the port 115, and the port 114 and chamber 132 are
connected to the low pressure return 137 through the valve 135.
Thus when retracting the piston 116 hydraulic fluid under pressure
is applied only to the right-hand face of the piston head 117, and
low pressure fluid from the interior of the piston rod 118 to the
right of the piston 127 is displaced as the piston rod 118 slides
relative to the piston 127, through the passage 123 of the element
122 to the low pressure return 137.
[0064] When the actuator of FIG. 3 is used to operate a thrust
reverser system of an aircraft gas turbine engine there is
sufficient force generated by the actuator, in the initial movement
of the actuator from its rest position, to start the deployment of
the thrust reverser cowls against the air/gas flow resistance
mentioned above. However, at a point at which this resistance
ceases, and the air/gas flow starts to assist deployment of the
cowls, the piston 127 will have been arrested by the shoulder 126
of the element 122 and thereafter the actuator will apply only
relatively small loading to the cowls in the deployment direction.
During this phase of the movement the primary purpose of the
actuator is to control the movement of the cowls and not
necessarily to drive them to their deployed position since the
driving force necessary to achieve deployment may well be derived
from the air/gas flow around the cowls. However, it will be
recognised that if the cowls need to be deployed while the
aeroplane is stationary, with the engine not operating, for example
for servicing, then there will be sufficient force generated by the
actuator, throughout the whole stroke of the actuator, to move the
cowls from their stowed to their deployed positions and back
again.
[0065] Should there be any leakage of hydraulic fluid past the
seals 128, 129 of the supplementary piston 127, then such leakage
will collect in the closed end of the piston rod 118 and will be
returned to low pressure 137 through the central passage 123 of the
element 122.
[0066] Turning now to the alternative construction illustrated in
FIG. 4, parts common to FIG. 3 carry the same reference numerals.
Moreover, although the piston 116 includes a piston head and piston
rod 118 equivalent to those of FIG. 3, the piston head of the
piston 116 of FIG. 3 is not visible in FIG. 4, because FIG. 4 is a
view of only part of the actuator. The actuator illustrated in FIG.
4 differs from the actuator described above with reference to FIG.
3 primarily in that the actuator of FIG. 4 incorporates a hollow
rotatable synchronising shaft 142 having a central passage 143
extending therethrough. As described hereinbefore, it is known in
other linear actuators to incorporate an axially fixed, but
rotatable synchronising shaft which is rotated through the
intermediary of a screw mechanism as the piston 116 moves axially
relative to the cylinder 111. The thrust reverser system of a gas
turbine engine may utilise a plurality of actuators to operate the
cowls, and of course it is desirable for the actuators to move in
synchronism so that the cowls are not subjected to unbalanced, or
twisting loads. In use the synchronising shafts of the actuators of
one or more cowls can be interconnected by a shaft arrangement so
that the pistons of the actuators are constrained to move in
unison.
[0067] The inventor has recognised that the presence of a
synchronising shaft 142 in the actuator shown in FIG. 4 removes the
need to provide the element 122 described above in relation to FIG.
3 since the synchronising shaft 142, although rotatable, is axially
fixed and can thus be used to support the supplementary piston 127.
In FIG. 4 it can be seen that the hollow synchronising shaft 142 is
adapted at its end remote from the cylinder closure member 112 to
slidably receive within it a supplementary piston 127. The
supplementary piston 127 protrudes from the interior of the shaft
142 at the free end of the shaft 142 and includes a piston head
127a slidably received within the piston rod 118 of the main piston
116. An annular seal 129 seals the sliding interface of the
supplementary piston 127 and the interior of the piston rod 118,
and the piston rod 118 includes an internal, radially inwardly
extending shoulder 131, against which the right-hand end of the
head 127a of the piston 127 can abut.
[0068] The supplementary piston 127 extends into the shaft 142 and
is slidably received therein. The left-hand end of the
supplementary piston 127 within the shaft 142 can abut a radially
inwardly extending shoulder 145 provided in the shaft 142 to define
the axial rest position of the supplementary piston 127 relative to
the shaft 142 and the cylinder 111. An annular sealing ring 128
seals the sliding interface of the left-hand end of the
supplementary piston 127 and the interior of the shaft 142.
Adjacent its left-hand end the supplementary piston 127 includes a
radially outwardly extending shoulder 127b which can abut a thrust
bearing assembly 146 carried at the free end of the shaft 142. The
thrust bearing assembly 146 is secured to the free end of the shaft
142 and extends radially inwardly to surround the supplementary
piston 127 between the piston head 127a and the shoulder 127b.
Abutment of the shoulder 127b with the bearing assembly 146 defines
the limit position of the supplementary piston 127 relative to the
shaft 142 and cylinder 111. The thrust bearing assembly 146 can
take a number of forms, but conveniently includes a plurality of
rotatable balls which ride on the outer surface of the piston 127
during axial movement of the piston 127 relative to the shaft 142,
and which abut the shoulder 127b in the limit position of the
piston 127. It will be understood that during axial translation of
the piston 116 relative to the cylinder 111 the shaft 142 is being
rotated within the piston rod 118. As the piston head 127a is,
during the initial part of the movement of the piston 116, bearing
against the shoulder 131 of the piston rod 118 then the piston 127
will be moving axially with the piston 116, but will not be
rotating with the shaft 142. Thus when the piston 127 reaches its
limit position with shoulder 127b abutting the bearing 146 the
bearing 146 will facilitate rotation of the shaft 142 about the
piston 127 so that the piston 127 is not caused to rotate in the
piston rod 118.
[0069] The operation of the actuator illustrated in FIG. 4 is
substantially identical to that described above in relation to FIG.
3. When the actuator is in its rest position the head 127a of the
supplementary piston 127 abuts the shoulder 131 of the piston rod
118 and the supplementary piston 127 is in a rest position relative
to the cylinder 111, said rest position conveniently being defined
by abutment of the left-hand end of the supplementary piston 127
with the shoulder 145 of the shaft 142. Application of hydraulic
fluid under pressure to the chamber 132 of the actuator applies
hydraulic pressure to the piston head, the piston shaft 118, and
the left-hand face of the piston head 127a of the supplementary
piston 127. Thus the supplementary piston 127 moves to the right
with the piston 116, providing part of the driving force for moving
the piston 116. The piston 116 moves to the right, that is to say
in an actuation direction, at least in part under the influence of
the piston 127 until the piston 127 reaches its limit position
defined by abutment of the shoulder 127b with the thrust bearing
146. Thereafter the piston 116 continues to move to the right
sliding relative to the piston 127. As the shaft 142 of the FIG. 4
arrangement is hollow, the passage 143 of the shaft 142 can provide
a low pressure return for hydraulic fluid leaking past the seal 129
of the piston 127 and for hydraulic fluid drawn into and expelled
from the piston rod 118 as it moves relative to the piston 127.
[0070] In both the FIG. 3 and FIG. 4 actuators it will be
appreciated that during retraction movement of the piston 116 there
is a point at which the shoulder 131 of the piston rod 118 abuts
the supplementary piston 127 which, at this time, is still in its
limit position. Thereafter the supplementary piston is returned to
its rest position by the retraction movement of the piston 116 to
its rest position. As mentioned above the rest position of the
piston 127 of FIG. 4 is conveniently defined by abutment of the
left-hand end of the piston 127 with the shoulder 145 in the shaft
142. Similarly, in FIG. 3 the rest position of the supplementary
piston 127 is conveniently defined by abutment of the left-hand end
of the piston 127 with the shoulder 125 on the element 122. It is
to be understood however that it is not essential that the rest
positions of the pistons 127 are defined by abutment, since the
pistons 127 will be carried back to a rest position by the
respective piston 116 during retraction.
[0071] Referring next to FIGS. 5 and 6 of the drawings, the linear
hydraulic actuator 211 includes an elongate hydraulic cylinder 212
of circular cross-section and an elongate piston assembly 213 also
of circular cross-section slidably received within the cylinder 212
and coaxial therewith.
[0072] The cylinder 212 is closed at one end by an end cap 214 and
closed at its opposite end by an annular bush 215 through which an
elongate hollow piston rod 216 of the piston assembly 213 extends.
At its outermost end the piston rod 216 is provided with a coupling
217 whereby in use the piston rod is connected to a component to be
moved by the actuator, for example a movable cowl of an aircraft
gas turbine engine thrust reverser arrangement. Seal rings 218
carried by the bush 215 seal the sliding interface of the bush 215
and the piston rod 216.
[0073] At its end within the cylinder 212 the piston rod 216
carries an annular piston head 219 of the piston assembly 213. The
outer diameter of the piston rod 216 is less than the inner
diameter of the cylinder 212 and at the innermost end of the piston
rod 213 the annular clearance between the rod 216 and the cylinder
wall is occupied by the annular piston head 219, a seal ring 221
carried by the piston head 219 serving to seal the sliding
interface of the piston assembly 213 in the cylinder 212.
[0074] Adjacent the end cap 214 the wall of the cylinder 212 is
formed with an inlet union 222 through which hydraulic fluid under
pressure can be admitted to a pressure chamber 225 defined at one
end of the cylinder between the end cap 214 and the annular end
face of the piston assembly 213. Adjacent the bush 215 the wall of
the cylinder 212 is formed with an outlet union 223 through which
hydraulic fluid can flow to and from the annular clearance between
the piston assembly 213 and the cylinder 212.
[0075] Extending coaxially within the cylinder 211, from the end
cap 214, is an elongate rod 226. The rod 226 is anchored to the end
cap 214 and extends within the hollow piston rod 216 of the piston
assembly 213. At its free end, remote from the end cap 214 and
adjacent the bush 215, the rod 226 carries a seal member in the
form of an external collar 227 slidably engaging the interior
surface of the piston rod 216, a seal ring 228 sealing the sliding
interface of the collar 227 and piston rod 216. A seal ring 229
seals the region of engagement of the rod 226 with the end cap 214.
It will be recognised that the rod 226, collar 227, and seal ring
228 isolate an inner, end chamber 231 defined within the piston rod
216 from the pressure chamber 225 of the cylinder 212. Thus
hydraulic fluid under pressure admitted to the chamber 225 through
the union 222 acts upon the annular end face 224 of the piston
assembly, but does not act upon the inner opposite end surface 232
of the piston assembly.
[0076] It will be appreciated that the arrangement of FIG. 5 is
very similar to that of FIG. 2, except that the chamber 231 is not
vented directly to the atmosphere.
[0077] In FIG. 5 the actuator is shown with the piston assembly in
a retracted position, and about to be operated to displace the
piston assembly 213 to the right in FIG. 5. The hydraulic fluid
pressure system associated with the actuator 211 includes a first
fluid line, identified in the drawings as HP, connected to the
output of a pump (not shown) for supplying high pressure (HP)
hydraulic fluid. A second fluid line, indicated in the drawings as
LP is the low pressure return line of the system, and may be
connected to an hydraulic fluid reservoir. The HP line is connected
through first and second restrictors 235, 236 in series to a line
233 in turn connected to the port 223 of the cylinder 212. The LP
line, and a tapping intermediate the restrictors 235, 236 are
connected to a change-over valve 234 and a supply line 237 from the
valve 234 is connected to the port 222. When it is desired to
extend the actuator from its retracted position the change-over
valve 234 is moved to the position shown in FIG. 5 in which the
tapping intermediate the restrictors 235 and 236 of the HP line is
connected through a further restrictor 238 to the supply line 237.
It will be recognised that in this position of the change-over
valve 234 both the pressure chamber 225, and the annular chamber
defined between the piston rod 216 and the inner wall of the
cylinder 212 are exposed to hydraulic fluid under high pressure
from the HP line. However, the area of the piston assembly 213
exposed to HP in the chamber 225 exceeds the area exposed to HP in
the annular gap between the piston rod 216 and the cylinder 212 by
the thickness of the piston rod 216, and thus there is a force on
the piston assembly 213 tending to displace the assembly to the
right in the drawings.
[0078] The elongate rod 226 is formed with a through passage or
conduit 239 communicating with the chamber 231 within the piston
rod 216. At its end remote from the chamber 231 the conduit 239
communicates with a line 241 which is connected to the line 237
through a restrictor 242. Thus not only is the chamber 225 supplied
with hydraulic fluid at high pressure, but also the chamber 231 is
supplied with hydraulic fluid at high pressure. However, in normal
operation, the speed at which the piston assembly 213 is moved
under the action of pressurised hydraulic fluid admitted to the
chamber 225 and thus the rate at which the volume of the chamber
231 increases, is greater than the rate at which hydraulic fluid
can be supplied through the restrictor 242 to the chamber 231.
During normal operation therefore fluid supplied to the chamber 231
through the restrictor 232 and conduit 239 does not provide a
driving force on the piston assembly 213, and, cavitation will
occur in the fluid within the chamber 231 as the volume of the
chamber 231 increases at a rate greater than that at which fluid is
supplied through the restrictor 242.
[0079] In the event that the actuator is required to drive a load
which exceeds that which can be moved by the hydraulic pressure
applied to the surface 224 of the piston assembly 213 then the
actuator will stall, that is to say the piston assembly will fail
to move, or will cease to move. In such circumstances the volume of
the chamber 231 will not be increasing, and so the pressure applied
to the chamber 231 will increase as fluid continues to flow through
the restrictor 242, climbing towards the pressure in the chamber
225, so that pressure acting on the internal surface 232 of the
piston rod 216 will assist pressure acting on the surface 224 of
the piston assembly providing the piston with a much increased
effective area and allowing the actuator to generate sufficient
force to overcome the load causing the actuator to stall.
Immediately the load has been overcome, and the force required of
the actuator falls, then the speed of movement of the piston
assembly 213 will increase, and the surface 232 will cease to be
effective as the rate of supply of fluid to the chamber 231 through
the restrictor 242 will be less than the rate at which the volume
of the chamber 231 is increasing, and again cavitation in the
chamber 231 will occur.
[0080] It will be understood from the foregoing description that
the actuator normally provides an output force determined by the
pressure in the chamber 225 acting on the surface 224 of the piston
assembly, but in a stall condition the pressure in the chamber 231
increases allowing the surface 232 to augment the surface 224 and
thus to provide the actuator with an increased output force.
[0081] If, in operation, it appears that cavitation within the
chamber 231 is problematic, then the line 241 can also be connected
to the LP line so that the LP line provides top-up fluid to the
chamber 231 to prevent cavitation. The connection between the line
241 and the LP line will of course include a non-return valve so
that fluid cannot flow from the line 241 to the LP line.
[0082] It will be recognised that during extension movement of the
piston assembly 213 fluid is displaced from the annular gap between
the piston rod 216 and the inner surface of the cylinder 212. This
fluid is displaced back along the line 233 through the restrictor
236 and augments the volume of fluid flowing through the restrictor
235 and the valve 234 to the port 222.
[0083] FIG. 6 illustrates the position of the valve 234 during
retraction movement of the actuator, when the piston assembly 213
is being driven to the left. It can be seen that the HP line is
still connected through the restrictors 235 and 236 and the line
233 to the port 223, but the line 237 is now connected to the LP
line. Thus high pressure fluid acting on the head 219 of the piston
within the annular gap between the piston rod and the cylinder
pushes the piston assembly 213 to the left and hydraulic fluid
displaced from the chamber 225 flows from the port 222, through the
line 237 and the change-over valve 234 to the LP line. At the same
time hydraulic fluid displaced from the chamber 231 by the
reduction in the volume of the chamber 231 accompanying leftward
movement of the piston assembly flows through the line 241 and a
non-return valve 243 in parallel with the restrictor 242, to the
line 237. The positioning of the non-return valve 243 is such that
it opens to allow flow from the chamber 231 back to the line 237
without the fluid being forced to flow through the restrictor 242,
but closes during extension of the actuator so that fluid from the
line 237 to the chamber 231 must flow through the restrictor
242.
[0084] It is believed that in practice it may be possible to
combine the restrictor 242 and the check valve 243 into a single
component which can be considered to be a "leaky" non-return valve.
Thus, the "leaky" non-return valve would be such that during
extension movement of the actuator there is a leakage flow through
the non-return valve which thus constitutes a restrictor, but
during retraction movement the check valve will open so that there
is little or no restriction upon the return flow from the chamber
231.
[0085] The modified actuator illustrated in FIG. 7 embodies a
so-called "leaky" non-return valve. In FIG. 7 components of the
actuator common to FIGS. 5 and 6 carry the same reference numerals.
The most significant difference between the actuator of FIGS. 5 and
6 and the modified actuator of FIG. 7, is that the conduit 239 in
the rod 226 of the FIG. 7 actuator is connected through a radial
drilling 245 in the rod 226, to the pressure chamber 225 of the
actuator and in place of the external parallel arrangement of
restrictor 242 and non-return valve 243 there is provided a
combined restrictor and non-return valve 246 housed within the
inner end of the rod 226 and providing a flow path between the
conduit 239 and the end chamber 231 of the actuator.
[0086] The combined restrictor and non-return valve 246 is a
"leaky" non-return valve, typically a ball-valve (as shown in FIG.
7) or a poppet-valve. The valve is "leaky" in that there is a
restricted, leakage path for fluid flow from the conduit 239 into
the chamber 231 between the spring pressed ball or the poppet of
the valve and its associated valve seating. Thus in the closed
position of the valve 246 the valve defines the equivalent of the
restrictor 242 of FIGS. 5 and 6. However, during retraction
movement of the piston assembly 213 of the actuator the valve 246
acts in the same manner as the valve 243 shown in FIGS. 5 and 6,
the ball 247 being lifted away from its seating, against the action
of its closure spring, to permit a relatively unrestricted flow of
hydraulic fluid from the chamber 231, through the conduit 239 and
the radial drilling 245 into the chamber 225, so as to flow with
the fluid being expelled from the chamber 225 through the line
237.
[0087] It will be recognised therefore that the operation of the
actuator illustrated in FIG. 7 is substantially identical to that
described above with reference to FIGS. 5 and 6.
[0088] In the embodiments described herein reference is made to
flow restrictors 235, 236 and 238. An understanding of the function
of the restrictors is not of importance to the present invention,
and it is sufficient to recognise that the restrictors are sized in
relation to one another to control the speed of actuation of the
actuator and to minimise cavitation during retraction movement of
the actuator, and also in deployment movement of the actuator where
deployment may be assisted by, for example, reversal of the axial
loading imposed on the piston assembly.
[0089] If desired, the embodiments of FIGS. 5 to 7 may be modified
to incorporate a synchronisation mechanism, for example of the
general type described hereinbefore. It may also be possible to
replace the axially fixed seal member with a movable seal member,
for example of the supplementary piston type described with
reference to FIG. 3 or FIG. 4.
[0090] It will be appreciated that a range of modifications and
alterations to the arrangements described hereinbefore without
departing from the scope of the invention.
* * * * *